WARNING🚨 Unfortunately, this can't be written in an exciting way😴
Main deduction card
Basically, you should use the main deduction card where you earn the most money (fx. work or study grants, "SU"). You should only use it one place. When a payer uses your main deduction card, you will only pay taxes of the amount money higher than your deduction. If your main deduction card is attached to multiple payers, your deduction will be too high and you won't pay enough taxes. In the end, you will get a tax shock. Therefore, it is a great idea to have your main deduction card attached to one place only.
Secondary tax card
The secondary tax card will only be used when having multiple payers (fx. an employer and SU). The main deduction card will show both your income tax rate and the deduction. When a payer uses your secondary tax card, the payer will take the full income tax rate of the payment. So, if your main deduction card is already attached to one payer, you should attach your secondary tax card elsewhere.
Tax exemption card
Your tax exemption card will be used, if your expected yearly income is less than what you are allowed to earn without paying taxes (usually, around DKK 42.000). Usually this happens, when getting your first job or living at home receiving SU without having a job.
You won't have to pay taxes before your yearly salary exceed this amount of money.
Even though, you have a tax exemption card, you will always pay 8 % of your salary in labour-market contribution. Your tax exemption card can be used by multiple payers. When it has been used, the payers will automatically pull the taxes with the rate of taxation noted on your estimate of future income.
Yes, this is surely boring to read, but yet important to understand. We hope that it answered your questions 😊
If you want to learn more about this subject, we highly recommend that you will take a look on this illustration that the Central Tax Administration has made: https://skat.dk/SKAT.aspx?oid=2247419.
Have a good day 👋🏼